According to a widespread grafting process, radical groups are introduced into the chain of the base polymer, e.g. by means of high-energy radiation, on which radical groups the grafting can take place by means of radical chain polymerization.
The generation of the activated, e.g. radical groups which effect the starting point of the grafting takes place in these grafting processes in an untargeted fashion, that is, the grafting does not take place exclusively at a certain grouping of the chain of the base polymer, such as e.g. on the nitrogen atom, but rather on all positions of the polymer chain which can be activated by means of high-energy radiation, e.g. also on methylene groups. A more or less strong degradation of polymer chains takes place at the same time by means of high-energy radiation and a damaging of the mechanical strength of the base polymer occurs as a consequence of the lowering of the degree of polymerization occasioned therewith. Likewise undesired side reactions in radiation-induced grafting are cross-linking reactions, which result in an embrittlement of the material.
Other processes of graft copolymerization are based on activation under radical formation by means of strong oxidizing agents such as e.g. Ce.sup.IV salts. This process can only be used in the case of very low pH'es, namely below pH=2, because otherwise a hydrolytic precipitation of the Ce.sup.IV salt occurs. The use of this process in the case of the polymers intended for the process of the invention results in a hydrolytic damaging of these polymers and a diminution of the mechanical strength due to a reduction in the degree of polymerization can also be observed. In addition, no purposeful grafting for increasing the chemical resistance of the base polymer is even possible according to this process.
Other processes for graft polymerization are based on chain transfer in that a homopolymerization of the monomer is induced by a radical initiator in the presence of the base polymer to be grafted, for which high temperatures must generally be employed (70.degree.-80.degree. C.). The grafting takes place in this instance by means of interaction of the growing polymer radical with the base polymer. The grafting site can also not be influenced in a purposeful manner in this instance. A further disadvantage of this grafting process resides in the fact that only a slight portion of the monomer used is consumed for the grafting whereas at the same time a considerable amount of homopolymer is produced. The formation of homopolymer is undesired because this increases the amount of the required monomer, which adversely affects the economy of the process and, moreover, a specific method step for removing the homopolymer becomes necessary.
The previously named grafting processes have the fact in common that the grafting does not take place at any sharply defined position of the base polymer, especially not with preference on the nitrogen atom. The chemical nature of the corresponding bonds such as the peptide group and the carbamic acid group is therefore not changed and a positive influence on the chemical stability of these bonds can therefore not take place.
It is also known that polyamides can be grafted with acrylamide or acrylonitrile in such a manner that in a first method step the hydrogen atoms on the nitrogen atoms are replaced by chlorine atoms. The halogen-substituted polyamides are then converted back into the initial polymer by means of hydrazine or iron-II-salts, during which time a radical transitional state of nitrogen appears. A radical chain polymerization on the nitrogen takes place in the presence of the named monomers as well as in the case of other redox-initiated polymerization processes. This reaction is described e.g. by K. V. Phung and R. C. Schulz in "Makromolekulare Chemie", 180, 1825 (1979). It was used in this paper to demonstrate the mentioned radical transitional state during the reduction.
Another paper which concerns grafting onto N-halogenated polyamides describes the initiation by means of metal carbonyls (C. H. Bamford, F. C. Duncan, R. J. W. Reynolds in "J. Poly. Sci." part C, pp. 419-432 (1968). These known processes are relatively expensive.
The present invention therefore has the object of creating a process for grafting nitrogen-containing polymers which is very simple to carry out and with whose aid it is possible to graft nitrogen-containing polymers in a uniform or purposeful fashion on the surface of form bodies in order to modify the properties of these nitrogen-containing initial materials in a desired manner.
The changes of the surface properties consist both in the increasing of the chemical resistance of the surface area, which should occur in every case, as well as in the influencing of other chemical and/or physical surface properties, especially of the wetting- and adsorption behavior without the disadvantages which occur in traditional grafting processes becoming active. These disadvantages are, explicitly expressed: Chain degradation and cross-linking reactions on the base polymer and/or a high amount of homopolymerizate as well as an unintended and/or uncontrollable progression of the grafting deep into the surface.
The invention therefore has the further object of creating a process which makes it possible in the case of form bodies which are not very compact, especially microporous membranes with a surface/mass ratio in a range of up to 50 m.sup.2 /g, to carry out the grafting selectively over the entire polymer matrix or to limit it to the externally located chain areas of the base polymer.
To the extent that it is a object of the invention to create a process for increasing the chemical stability of the base polymer, especially in the surface area of form bodies, an increase of the resistance to chain degradation by means of oxidative and hydrolytic degradation as well as by radiation damage is to be understood thereunder. In particular, the problem of the process of the invention consists in converting the most labile groupings in the main chain of the base polymers such as the peptide- or carbamic acid group in the surface area into a form which is less susceptible to chemical attack in order to avoid a chain degradation of the base polymer. A stabilization of the surface layer should also protect the non-stabilized areas of the base polymer located thereunder from chemical attack. As a result thereof, it is not necessary to stabilize the entire base polymer of the form body, so that its mechanical properties are not altered in this area. An essential part of the problem definition is the fact that the above-named stabilization effects are achieved without the use of stabilizers which can be extracted with solvent.
A further object of the invention is a process for the production of composite bodies in which the form body used for grafting is provided on the surface with a layer of the graft polymer which is chemically connected to the form body and is essentially free of individual chains of the base polymer, so that the swelling properties of the layer of the graft polymer are exclusively determined by the type of the monomer used for the grafting and thus differ in a characteristic manner from those of the base polymer. In particular, graft polymers are to be understood thereunder which exhibit a high swelling capacity in aqueous media so that they are accessible, when they are provided with chemical groups which enable them to reversibly or irreversibly bond certain target substances, to these chemical groups for the particular target substances even in the interior of this layer. The target substances can be e.g. proteins, the groups capable of reversibly bonding can be ionic groups or affinity ligands, the groups capable of irreversibly bonding can be groups which can enter chemical bonds with amino- or sulfhydryl groups of proteins under mild conditions and are known in the state of the art. As a result of the fact that not only the surface but also the interior of the grafted polymer layer is accessible for the target substances, an especially high bonding capacity of the composite material should be achieved.
Whereas the areas of application for the above-named composite materials are in the area of the adsorptive separation of substances, a further object definition of the invention concerns the textile sector. The goal in the production of the composite bodies is here to provide formed bodies, especially textile fibers, with a grafted polymer layer which differs as regards the dyeing technology from the base polymer in the desired manner. An example for such an instance of application is present when the grafted-on polymer layer is to be dyed by means of a class of dyes for which the base polymer exhibits either no or only a slight affinity. This is especially desirable from the standpoint of textile technology when mixed fibers are to be dyed in one work step, e.g. mixed fibers of polyamides and cotton in a dye bath with reactive dyes for cellulose.
A further object of the invention is to create a process for the purposeful changing of the wetting properties of formed bodies, especially in the direction of an increase in the water wettability as well as the wettability by liquid with an even greater suface tension that that of water such as e.g. electrolytic solutions in high concentration. This goal is significant in all previously named areas of application. In addition to an increasing of the water wettability, there is also the problem of creating a process for the reduction of the adsorption capacity for lipophilic substances. In the case of microporous membranes primarily but not exclusively the protein adsorption should be reduced and in the case of textile fibers the contamination by fatty substances. In both cases a consequence of the reduction of the affinity for lipophilic substances is the fact that when such an adsorption has nevertheless taken place, it can be readily be undone again. In the case of textile fibers, this is expressed by the fact that washing can take place under considerably milder conditions than without such a modification of the surface. Likewise, filter membranes can be washed free again more easily after clogging by means of such a modification.
Another object of the invention is to make possible a process for the influencing of the zeta potential either in the direction of a negative or of a positive potential. The zeta potential also has considerable influence both in the case of filter materials and in the case of textiles on the properties of use as it determines the contamination properties in accordance with the contacting media. The electrostatic charge of the formed bodies is also closely associated with the zeta potential, which charge should also be reduced by the process of the invention in that the surface conductivity is increased by means of the introduction of ionic groups into the surface.
A further object of the invention consists in the case of form bodies which are not very compact and in the case of which the totality of the base polymer is located in a layer close to the surface in converting this form body entirely into a graft copolymer, during which an isotropic growth of this form body occurs with retention of its original form and the grafted form body differs in its chemical and optionally also mechanical properties and/or solubility properties in a desired manner from the initial product. In addition to a change in the chemical stability, these property changes can consist in an elevated as well as in a reduced solubility in certain solvents. As regards the mechanical properties, both an increase in the mechanical strength as well as an increase in flexibility can be achieved.